The research and development leading to the subject matter disclosed herein was not federally sponsored.
This invention relates to the production of polyethers, and in particular to a method for purifying a polyether to recover the polymerization catalyst therefrom.
Polyethers are high volume chemical compounds that are used in a wide variety of applications including, for example, the preparation of polyurethanes, making aminated polyethers, and surfactants. A common method of making polyethers is to polymerize an alkylene oxide in the presence of an xe2x80x9cinitiator compoundxe2x80x9d and an alkali metal hydroxide catalyst. In this way, polymers of the alkylene oxide can be prepared having a wide variety of molecular weights. The function of the initiator compound is to set the nominal functionality (number of hydroxyl groups per molecule) of the polyether. Sometimes the initiator provides the polyether with certain desirable functional properties. This is true, for example, in the case of many surfactants.
At the conclusion of the polymerization reaction, it is usually necessary to remove the alkali metal catalyst. Various methods for accomplishing this are known in the art. For example, the polyether can be mixed with water and a water-immiscible solvent and then subjected to electrostatic coalescence. The catalyst preferentially migrates to the water phase and the polyether migrates to the solvent phase, thereby achieving a separation of the catalyst from the polyether. However, that method has the substantial drawback of using an organic solvent which in addition to being an additional expense must be removed from the polyether in a separate processing step.
In a variation of the foregoing method, an acid is also added to the crude polyether before electrostatic coalescence or centrifugation. That process still has the problems associated with the use of the solvent. Additionally, the alkali metal catalyst is neutralized by the acid, forming salts that create a separate disposal problem.
Another approach to purifying polyethers is by treatment with an absorbent such as synthetic magnesium silicate and water. The absorbent, which contains the catalyst, is then separated from the polyether and the polyether is subsequently stripped of the water using heat and vacuum. This process is therefore energy-intensive and the catalyst is lost with the absorbent. A variation of this method, in which the water/adsorbent/polyether mixture is treated with carbon dioxide followed by filtration and water stripping, has the same disadvantages.
Ion exchange techniques have also been used to purify polyethers. These techniques have the disadvantages of requiring a solvent to reduce viscosity and the need to regenerate the ion exchange resin. Moreover, the catalyst is lost in the process and must be disposed of.
Another method of purifying polyethers involves mixing the crude polyether with water to form an emulsion, heating the emulsion to about 90xc2x0 to about 150xc2x0 C., and then passing the emulsion through a coalescing medium by which the emulsion is broken and separate aqueous and polyether phases are formed. Salts or additional alkali metal hydroxides may be added with the water to promote a greater density difference between the aqueous and polyether phases, thereby facilitating their separation.
Although the coalescing process just described provides certain advantages over other methods, it is desirable to improve the efficiency of the method in order to maximize the amount of alkali metal catalyst that is removed.
The most common catalyst for polyether production is potassium hydroxide (KOH). However, when KOH is used to polymerize propylene oxide to form a polyether, a significant level of monofunctional impurities often are formed by the isomerization of propylene oxide into allyl alcohol. The allyl alcohol acts as a monofunctional initiator for the polymerization of polyether monoalcohols. One method of reducing this undesired isomeration reaction is to use cesium hydroxide (CsOH) as the polymerization catalyst. However, CsOH is many times more expensive than KOH. Hence, for a process using CsOH to be economically viable, it is necessary to minimize the amount of catalyst that is lost during the polyether production.
Accordingly, it would be desirable to provide a process whereby an alkali metal hydroxide can be recovered from a crude polyether, which process does not require the use of organic solvents or absorbents, and which permits for minimal losses of the polyether polymerization catalyst.